1. Field of the Disclosure
The present disclosure relates to improved methods and systems for coating materials as well as improved protective coatings for materials. Particularly, the present disclosure is directed to methods and systems for making coatings including cobalt, phosphorous and particles of material having superior tribological characteristics.
2. Description of Related Art
Electroplated hard chrome coating is widely used as a wear resistant coating to prolong the life of mechanical components. However, conventional hard chrome electroplating processes generate hexavalent chromium ion which is a known carcinogen. Hence, there is a major effort throughout the electroplating industry to replace hard chrome coatings with an environmentally benign, non-carcinogenic coating having characteristics similar or superior to those of hard chrome.
Thermal spray hard coatings of chromium carbide, tungsten carbide, tribaloy, aluminum oxide and the like, using Plasma Spray, High Velocity Oxy Fuel (HVOF) and other similar processes are currently being used to replace hard chrome coatings. However, these processes have not been able to be used for non line of sight (NLOS) applications, such as the inner diameter (ID) of cylinders, bearing cavities and the like. Even for the outer surface applications, thermal spray coatings are generally deposited in thick layers and later ground to a desired thickness. Hence, thermal sprayed coatings are generally more expensive than electroplated hard chrome.
For NLOS applications, a number of electroplated coatings have been evaluated. These include electroplated Ni—P and Ni—W alloy coatings, Ni—SiC electrocomposite and other similar coatings. However, none of these coatings have all the desired characteristics of hard chrome. Also, nickel base coatings are now considered undesirable because it has been found that in some cases they can cause severe allergic reactions.
Recently, a new nanocrystalline Co—P base coating has been developed by pulse plating processes. The resulting nanocrystalline Co—P coating appears to be a very promising replacement for hard chrome as its characteristics are either equal or superior to those of hard chrome. However, the electroplating process for this nanocrystalline Co—P base coating is based on pulse plating. In pulse plating, the applied voltage between the anode and cathode is pulsed at different amplitudes and at various frequencies. This pulse plating process used to produce nanocrystalline Co—P coatings requires special power supplies which are currently available only for laboratory research and development. Large scale affordable pulsed power supplies for the production environment are not currently available.
In another aspect, engineered components are typically coated to provide wear resistance, corrosion resistance, and oxidation resistance of the base material. The most widely used coating is hard chrome because of its excellent wear resistance, low cost and ease of application. Hard chrome also has its own deficiencies, among them is adverse effects on the fatigue strength of base materials. Because of this undesirable impact on fatigue resistance or a “fatigue debit”, base materials are appropriately derated requiring heavier cross sections, weight and cost penalty. Fatigue debit results from micro cracks inherent in the hard chrome deposition that act as crack initiation sites. As a result, engineers have to design plated hardware susceptible to fatigue accordingly. This usually requires larger, stronger cross sections to compensate for the fatigue debit induced by the plating.
Hard chrome is not the only coating that creates a “fatigue debit.” Electroless nickel and High Velocity Oxy Fuel (HVOF thermal spray) coatings also have “fatigue debits”. High phosphorous electroless nickel has an amorphous, glassy brittle micro-structure, which is susceptible to cracking which causes a fatigue debit, while mid phosphorous electroless nickel and HVOF coatings contain porosities in their coatings. The not fully dense structure is similar to a cracked structure resulting in fatigue debit. Table A demonstrates typical fatigue debit data for electroless nickel and hard chromium.
TABLE ATypical “Fatigue Debit” results forHard Chromium and Electroless NickelBar, Coating TypeAvg. Fatigue CyclesFatigue Debit, %Uncoated 4130*117,426 (4 pcs) —Hard Chromium34,370 (3 pcs)70%(0.002″)Uncoated 4130*75,185 (2 pcs)—Hard Chromium27,468 (3 pcs)64%(0.002″)Uncoated 4130*156,252 (3 pcs) —Electroless Nickel85,019 (3 pcs)46%Uncoated 413075,185 (2 pcs)—Cr over Electroless39,059 (3 pcs)48%Ni*Different manufactured lots of fatigue specimens give different baselines for fatigue
One solution to counter the “fatigue debit” of plated coatings often used on fatigue sensitive hardware is the use of shot peening. Shot peening imparts a compressive stress on the surface of the material to be coated which retards the initiation of fatigue cracks. The need to shot peen hardware which will be electroplated adds expense to the overall manufacturing process. Therefore, there exists a need for coatings used on engineered stress-bearing components which provide wear and corrosion resistance without a “fatigue debit” of base materials.
As set forth above, there is a continued need for improved coatings and associated processes for replacing hard chrome. The present disclosure provides a solution for these and other problems.